Broadband waveguide transition for a centipede type traveling wave tube



D. K. WINSLOW 3,532,926

BROADBAND WAVEGUIDE TRANSITION FOR A CENTIP TYPE TRAVELING WAVE TUBE FiledMay 20, 1968 EDE FIG.|

|.8 INVENTOR.

{.2 DONALD K. VIINSLOW I22 BY I f 52-00 I 3400 5600 f L I -126M ATTORNEY United States Patent US. Cl. 315-3.5 3 Claims ABSTRACT OF THE DISCLOSURE A microwave tube is disclosed. The microwave tube includes an electron gun for forming and projecting a beam of electrons over an elongated beam path to a beam collector at the terminal end of the beam. A centipede type slow wave circuit is arranged along the beam path for electromagnetic interaction with the beam to produce an output microwave signal. A broadband waveguide transition is provided at the downstream end of the centipede slow wave circuit for extracting the output microwave signal from the circuit and for transmitting same to a suitable utilization device or load. The broadband waveguide transition includes an output ridged rectangular waveguide disposed in wave energy communication with the downstream end of the centipede slow Wave circuit via the intermediary of a centrally apertured conductive wall which partitions off the downstream end of the slow wave circuit. A conductive post extends from the ridged portion of the waveguide into the coupling hole. The ridged waveguide is tapered in height from essentially full height to a relatively short height at the post and coupling hole for providing a broadband impedance match between the output waveguide and the centipede slow wave circuit. The ridge is also tapered in height with increasing height taken in the direction toward the shallow end of the transition waveguide. Y

DESCRIPTION OF THE PRIOR ART Heretofore, microwave tubes have been constructed which have employed an output slow wave circuit comprised of an array of cavity resonators having negative mutual inductive coupling therebetween. Examples of such negative mutual inductive coupled cavities include the cloverleaf structure and the centipede slow wave structure described and claimed as equivalents in US. Pat. 3,233,139 issued Feb. 1, 1966 and assigned to the same assignee as the present invention; Output waveguide transitions have been developed for such negative mutually inductively coupled slow wave circuits. Such transitions have included a hollow cylindrical conductive post projecting from a broad wall of the output waveguide coaxially of the beam and coaxially of a wave energy coupling hole communicating through the end wall of the terminal cavity of the coupled cavity circuit. Morespecifically, the end wall of the terminal cavity of the slow wave circuit was provided with an enlarged central aperture coaxially aligned with the beam path. The conductive post extended from the shallow end of a tapered height hollow rectangular Waveguide coaxially of the beam hole and terminated substantially in the plane of the coupling hole. The output coupling hole was made approximately /2 a wavelength in diameter and was of a circular shape. The conductive post was cylindrical and the shallow end of the hollow rectangular waveguide was shorted a wavelength from the center of the conductive post. Such an output waveguide transition is disclosed and claimed in US. Pat. 3,374,390 issued Mar. 19, 1968 and assigned to the same assignee as the present invention.

Although the aforedescribed prior art waveguide transi- 3,532,926 Patented Oct. 6, 1970 tion provided an extremely wide band match between a cloverleaf type coupled cavity slow wave circuit and an output waveguide, such an output transition does not have sufiicient bandwidth to provide a match to the wider useable bandwidth of a centipede type coupled cavity slow wave circuit.

Therefore a need exists for an improved waveguide transition for coupling output wave energy from a centipede type coupled cavity slow wave circuit to an output waveguide.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved broadband waveguide transition for a centipede slow wave circuit and tubes using same.

One feature of the presentinvention is the provision, in a broadband waveguide transition from a centipede slow wave circuit to an output waveguide, of a ridged hollow transition section of output waveguide having a conductive post extending from the ridge into the mouth of a wave energy coupling hole communicating between the terminal cavity of the slow wave circuit and the waveguide, whereby a broadband impedance match is obtained between the centipede slow wave circuit and the output waveguide.

Another feature of the present invention is the same as the preceding feature wherein the hollow ridged transition section of output waveguide is tapered in height with the shallow end of the ridged waveguide terminating adjacent the conductive post which couples wave energy through the coupling hole in the terminal cavity of the centipede slow wave circuit.

Another feature of the present invention is the same as the preceding feature wherein the ridge portion of the transition waveguide is tapered in height with the highest portion of the ridge being disposed adjacent the conductive coupling post.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal view, partly in section and partly schematic, depicting a microwave tube incorporating features of the present invention,

FIG. 2 is an enlarged longitudinal sectional view of a portion of the structure of FIG. 1 delineated by line 22.

FIG. 3 is a transverse sectional view of the structure of FIG. 2 taken along lines 33 in the direction of the arrows, and

FIG. 4 is a plot of voltage standing Wave ratio vs. frequency which compares the bandpass characteristics of an output waveguide transition employing a rectangular waveguide and a ridged waveguide, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a microwave tube 1 incorporating features of the present invention. The tube 1 includes an electron gun 2 disposed at one end of the tube for forming and projecting a beam of electrons over an elongated beam path 3 to a beam collector structure 4 disposed at the terminal end of the beam path 3. An interaction circuit 5 is disposed along the beam path intermediate the gun 2 and the collector 4 for electromagnetic interaction with the beam to produce output microwave energy which is extracted from a downstream end of the interaction circuit 5 via an output waveguide 6 and fed to a suitable utilization device or load, not shown.

The interaction circuit 5 may comprise a single section of centipede type slow Wave circuit or may comprise at least two circuit sections, namely, a buncher circuit section 7 and an output circuit section 8. In the later case, the output circuit section 8 would comprise a centipede slow wave circuit and the buncher circuit 7 may comprise either a severed section of centipede slow wave circuit or some other type of interaction circuit, such as a succession of klystron type cavities tuned near the bandedges of the operating band of the tube. Microwave amplifier tubes utilizing a buncher circuit 7 followed by an output circuit 8 wherein the output circuit comprises a succession of negative mutual inductively coupled cavity resonators is disclosed and claimed in the aforecited U.S. patent 3,374,390.

The centipede type slow wave circuit is one type of negative mutual inductive coupled circuit as disclosed in the aforecited US. Patent 3,233,139. This type of circuit is also described in an article titled A Structure, Using Resonant Coupling Elements, Suitable for a High-Power Travelling-Wave Tube by A. F. Pearce, appearing in the Proceedings of the Institution of Electrical Engineers, vol. 105, Part B, Supplement II, London, May 1958, pp. 719- 726. The centipede slow wave circuit has the advantage of providing extremely wide bandwidth while being capable of operating to very high power levels with reasonable efficiency.

In operation, an input microwave signal to be amplified is fed to the upstream end of the buncher circuit 7 via input coaxial line 11. Input signal energy bunches the beam throughout the buncher circuit 7 and the bunched beam passes into the output circuit 8 wherein it excites a wave on the output circuit which cumulatively interacts with the beam to produce an amplified output signal. The output microwave signal is extracted from the downstream end of the output circuit 8 and fed via waveguide 6 and a gas tight wave permeable window 12 to a suitable load. A beam focus solenoid 13 coaxially surrounds the tube to produce an axially directed beam focusing magnetic field over the length of the beam path 3 for confining the electrons to the desired beam path.

Referring now to FIGS. 2 and 3, the novel broadband waveguide transition from the centipede slow wave circuit 8 to the output waveguide 6 will be more fully disclosed. Briefly, the centipede slow wave circuit 8 includes a hollow tubular conductor 14, as of copper, having an array of centrally apertured conductive disc structures 15 axially spaced apart along the length of the tube 14 and transversely mounted therein to define a succession of coupled cavity resonators 16 defined by the spaces bounded, on the sides, by the inside wall of the tube 14 and, on the ends, by adjacent conductive disc structures 15.

The conductive disc structures 15 are conveniently formed by a pair of similarly shaped disc structures 17 and 18, respectively. Each of the disc structures 17 and 18 includes a central ring portion and an outer periphery formed by an array of radially directed legs. The two discs 17 and 18 are placed together with all of the legs of one of the discs being formed into an S shape and all of the legs of the other disc being formed into a reverse S shape. The two central ring portions of the disc structures are then brazed together. The legs are interditated to provide negative mutual inductive coupling loops communicating between adjacent cavity resonators 16. A coolant jacket 21 having coolant passageways therein surrounds the tube 14 for carrying away heat generated in the RF. structure and which is conducted to the jacket via the discs 15 and tube 14.

A circular conductive plate 22, as of copper, closes off the downstream end of the coupled cavity circuit 8. A transition section of the output waveguide structure 6 is formed in the conductive disc 22 with the axis of the transition section of waveguide 6 being transverse to the axis of the beam path 3. The waveguide 6 transition is tapered in height with the shallow end of the waveguide 6 being disposed approximately on the beam axis. In addition, the output transition waveguide 6 is provided t a ntral r dge 23 which projects from a bottom 4 broad wall of the transition waveguide toward the upper broad wall thereof.

A hollow cylindrical conductive post 24, as of copper, surrounds the beam path 3 and projects from the ridge 23 toward the upper opposed broad wall of the guide 6 and also toward the terminal cavity of the slow wave circuit 8. A wave energy coupling hole 25 is provided in the upper wall of the waveguide in coaxial alignment with the beam 3 and the conductive post 24. Beam coupling hole 25 is elongated in the direction perpendicular to the plane of the narrow walls 26 of the output waveguide 6. In addition, the coupling hole 25 is coaxially aligned with the conductive post 24.

The innermost end of the waveguide 6 is closed off by a conductive shorting plug structure 28 which forms an inner end wall for the ridged waveguide 6. The conductive plug structure 28 includes a pair of spaced rectangular plug portions which fill the portions of the ridged waveguide 6 between the sidewalls 26 and the center ridge 23. In addition, a conductive plate 30 bridges between the pair of rectangular plugs to fill the region of the ridged waveguide between the ridge and the uppermost wall. The inner shorting face 29 of the two rectangular plug portions of the shorting plug structure 28 is slanted into the guide in such a manner as to deflect wave energy traveling in the guide 6 through a angle between the coupling hole 25 and the ridged waveguide 6. The bridging plate-like portion 30 of the conductive plug structure 28 is cut in an arc shape 31 to conform to the outer cylindrical surface of the adjacent conductive post 24. The conductive plug structure 28 is disposed generally in the region of the lip portion of the coupling hole 25.

The transition section of the output waveguide 6 is tapered in height from standard height WR284 hollow waveguide at 6' to a shallow height ridged waveguide at 6" near its terminal end defined by the shorting plug structure 28 disposed adjacent the transition post structure 24. The tapered height waveguide is provided for matching the characteristic impedance of the standard height rectangular waveguide, about 35012 at 6', to the characteristic impedance of the centipede slow wave circuit, an impedance of about 129. In addition, the height of the ridge 23 is tapered from zero height at 6 to approximately of the full height of the ridged waveguide at 6". This tapering of the ridge height and of the height of the waveguide 6 takes place over a transition length 35 of waveguide 6 which is approximately two wavelengths long in order to provide a broadband impedance match. Use of the ridged waveguide greatly increases the bandwidth of the impedance match between the output waveguide and the terminal cavity 16 of the centipede slow Wave circuit.

A graph of VSWR in the output transition section of waveguide at 6' versus frequency is shown in FIG. 4. The graph compares the bandpass characteristics of the output waveguide transition employing a tapered height section of hollow rectangular waveguide with and without the provision of the ridge 23. As seen from the graph, provision of the ridge 23 increased the bandwidth at the high frequency end of the band from approximately 3350 mHz. to approximately 3650 mI-Iz.

In operation, output wave energy on the centipede slow wave circuit 8 reaches the terminal cavity section 16 and is coupled through the output coupling hole 25 down along the short section of coaxial line formed by the post 24 and the adjacent conductive wall structure including the edge of the coupling hole 25. The wave energy then is deflected through a 90 bend, by plug 28, and travels out the ridged waveguide 6 to a suitable load.

Since many changes could be made in the above constructionand many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a traveling wave tube, means for forming and projecting a beam of electrons over an elongated beam path, means at the terminal end of the beam for collecting and dissipating the energy of the beam, means forming a centipede slow wave circuit disposed along the beam path for cumulative electromagnetic interaction between the wave traveling on said slow wave circuit and electrons of the beam to produce output microwave energy on said slow wave circuit, means forming a hollow waveguide for transmitting the output microwave energy to a load, means forming a waveguide transition interconnecting said slow wave circuit and said hollow waveguide for matching the impedance of said slow wave circuit to the impedance of said waveguide over a broadband of frequencies, the improvement wherein, said waveguide transition means includes a tapered section of hollow Waveguide having a cross sectional dimension which progressively decreases taken in the direction from said output hollow waveguide toward said centipede slow wave circuit, and disposed within said tapered section, a conductive ridge projecting into said waveguide from an inside wall thereof, said ridge being elongated and extending in a direction of power flow through said tapered section of waveguide for providing a relatively wideband impedance match to said slow wave circuit.

2. The apparatus of claim 1 wherein said conductive ridge portion of said transition section of waveguide is tapered having a height which increases in a direction taken from said output hollow waveguide toward said centipede slow wave circuit.

3. The apparatus of claim 1 wherein said tapered section of waveguide includes a hollow conductive post coaxially disposed of the beam path with the beam path extending axially through said post, said post projecting from said ridge portion of said tapered waveguide section toward the terminal end of said centipede slow wave circuit, and said conductive post extending into a region of space adjacent the terminal end of said centipede slow wave circuit for coupling the wave energy from said centipede slow Wave circuit into said ridged transition section of waveguide.

References Cited UNITED STATES PATENTS 2,886,742 5/1959 Hull 315-3953 X 3,233,139 2/1966 Chodorow 315- 3,441,783 4/ 1969 Harris et a1 3153.5 X

HERMAN K. SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner US Cl. X.R. 

